This invention relates to magnetic recording media having excellent magnetic recording characteristics over a wide frequency bandwith, and more specifically to magnetic recording media improved in their running performance and durability.
Reflecting the recent volume increase of information and the recent great improvements in frequency characteristics and the like of recording and reproducing systems, magnetic recording media are also desired strongly to have expanded recording bands.
It has been the well known that conventional magnetic recording media have inherent limitations with respect to their characteristics in the short wavelength range, because such conventional magnetic recording media make use of their lengthwise remanent magnetization and their intra-medium self-demagnetizing fields tend to increase and their magnetization vectors are caused to turn so as to minimize their intra-medium energy as recording wavelengths become shorter, resulting in the occurrence of abrupt output drops.
The most general technique for minimizing the influence of such a self-demagnetizing field as much as possible is to increase the coercive force of each medium so that it can overcome the self-demagnetizing field. However, this manner of solution induces a reduction in the penetration of magnetization in the thicknesswise direction of the medium, thereby causing long wavelength outputs to drop or requiring erasing and recording heads which have very high magnetic flux densities and, correspondingly, leading to increases in the prices of heads in addition to the cost of media.
In order to solve such problems as mentioned above, it has been proposed by the present inventors to use a magnetic recording medium having a double-layered structure obtained by successively forming, on a non-magnetic substrate, a first magnetic layer having an easy axis of magnetization in the in-plane direction of the substrate and a second magnetic layer having an easy axis of magnetization in a direction perpendicular to the plane of the substrate in which the second magnetic layer contains, as its principal component, a hexagonal magnetic powder material such as barium ferrite magnetic powder, as disclosed in Japanese Patent Application No. 1545/1982, filed Jan. 8, 1982, and published July 16, 1983, as Japanese published patent application No. 119610/1983.
A magnetic recording medium having such a double-layered structure is produced by successively forming, on the surface of a substrate made for example of polyethylene terephthalate resin, a first magnetic coating layer and second magnetic coating layer by applying their corresponding magnetic coating formulations which are each prepared by dispersing desired magnetic powder together with a dispersant in a resinous binder.
However, the running performance of the above magnetic recording medium was not always satisfactorily smooth, though good in recording characteristics, when it was formed into tapes or discs and allowed to run in actual equipment. Its durability was also insufficient, presumably because of its poor running performance.
Such poor running performance seems to be attributed to two causes; one of which is frictional resistance, which occurs between the second magnetic layer of a tape or disc made of the magnetic recording medium and various parts and members of the actual equipment when the former is allowed to run in contact with the latter, and the other of which is electrification phenomenon which occurs, as a result of the above-mentioned frictional contact, in the second magnetic layer which is an insulator. Especially, the latter (electrification phenomenon) seems to serve as a dominant cause.
It has been attempted to enhance the lubricity of the second magnetic layer by incorporating a trace amount of a lubricant (for example, silicone oil, fatty acid ester, or the like) in the above-described magnetic coating formulation for the second magnetic layer with a view toward improving the running performance by removing the former cause (frictional resistance).
However, the above attempt still allowed the latter cause (electrification phenomenon) to remain because components of the second magnetic layer were insulators respectively. Thus, the tape was caused to stick on cylinders and other parts of an actual equipment because of the electrification phenomenon which occurred when the tape was running or the back face of its substrate and its second magnetic layer were subjected to the "sticking" phenomenon when fast-forwarded or rewound, whereby developing the so-called wound-up wrinkles or pleats when the tape was even a little slack.
It is routinely practiced, as a first countermeasure against the above problems, to incorporate a conductive material such as carbon black additionally in the second magnetic layer so as to impart a suitable degree of conductivity thereto, whereby to reduce the frictional electrification. This countermeasure is certainly effective to remove the electrification phenomenon to a satisfactory extent and thus to improve the running performance. However, it lowers reproduction outputs in the high band (short wavelength) range which is considered to be most critical, resulting in an increased output loss. Moreover, the magnetic powder (e.g., generally, barium ferrite) of the second magnetic layer has higher insulating capacity, compared with the magnetic powder (for example, .gamma.-ferrite, Co-substituted .gamma.-ferrite or chromium oxide) of the second magnetic layer. Thus, it is necessary to incorporate a conductive material in such a large amount that overcomes the insulation of the magnetic powder, thereby further aggravating the above-described inconvenient deterioration in characteristics.
A second countermeasure is to form a conductive layer having a suitable degree of conductivity over the back face of a substrate. This eliminates the above-mentioned "sticking" phenomenon between the back face of the substrate and the second magnetic layer. Although it is easy to apply two or more coating layers, one over another, on one side of a substrate in one step by making a small improvement to the production facilities, twice as many steps are required to form coating layers over respective sides of a substrate. If one wants to carry out the application of coating layers over respective sides of a substrate in one step, a huge investment would be indispensable on production facilities. In either case, the production cost of the thus-obtained product will be high. Thus, the above production processes are extremely disadvantageous from the economical standpoint.